In the completion and operation of oil wells, gas wells, water wells, and similar boreholes, it frequently is desirable to alter the producing characteristics of the formation by treating the well. Many such treatments involve the use of particulate material. For example, in hydraulic fracturing, particles (propping agents or proppants) are used to maintain the fracture in a propped condition. In hydraulic fracturing, propping agent particles under high closure stress tend to fragment and disintegrate. At closure stresses above about 5000 psi, silica sand, the most common proppant, is not normally employed due to its propensity to disintegrate. The resulting fines from this disintegration migrate and plug the interstitial flow passages in the propped interval. These migratory fines drastically reduce the permeability of the propped fracture.
Other propping agents have been used to increase well productivity. Organic materials, such as the shells of walnuts, coconuts and pecans have been used with some success. These organic materials are deformed rather than crushed when a fracture closes under the overburden load. Aluminum propping agents are another type of propping agent that deforms rather than fails under loading. While propping agents such as these avoid the problem of creating fines, they suffer the infirmity of allowing the propped fracture to close as the proppant is squeezed flatter and flatter with time. In addition, as these particles are squeezed flat the spaces between the particles grow smaller. This combination of decreased fracture width and decreased space between the particles results in reduced flow capacities.
An improved proppant over the materials mentioned above is spherical pellets of high strength glass. These high strength glass proppants are vitreous, rigid and have a high compressive strength which allows them to withstand overburden pressures of moderate magnitude. In addition, their uniform spherical shape aids in placing the particles and providing maximum flow through the fracture. While these beads have a high strength when employed in monolayers, they are less satisfactory in multilayer packs. In brine at 250.degree. F., the high strength glass beads have a tendency to disintegrate at stress levels between 5000 and 6000 psi with a resultant permeability which is no better, if not worse than sand under comparable conditions.
Resin coated particles have been used in efforts to improve the stability of proppants at high closure stresses. Sand or other substrates have been coated with an infusible resin such as an epoxy or phenolic resin. These materials are superior to sand at intermediate stress levels. However, at high temperature and high stress levels, the resin coated particles still show decrease in permeability to about the same degree as silica sand.
U.S. Pat. No. 3,492,147 to Young et al. describes a process for producing particulate solid coated with an infusible resin. The particulates to be coated include sand, nut shells, glass beads and aluminum pellets. The resins used include urea-aldehyde resins, phenol-aldehyde resins, epoxy resins, furfuryl alcohol resins and polyester or alkyd resins. These particles are used as proppants in fracturing operations.
U.S. Pat. No. 4,443,347 also describes a method for propping fractures in subterranean formations using proppants comprised of sand particles with a precured phenol formaldehyde resin coating.
U.S. Pat. No. 3,929,191 to Graham et al. discloses a method for producing coated particles for use in treating subterranean formations. Particles in this method are coated with a resin dissolved in a solvent which is then evaporated. The patent also discloses the coating may be produced by mixing the particles with a melted resin and subsequently cooling the mixture, forming a coating of resin on the particles. The Graham patent also discloses that the addition of coupling agents to the system improves the strength of the resin-substrate bond.
Although self consolidating resin coated sands have proven satisfactory in numerous applications, concern exists over their use under high closure stresses. As the self consolidating resin coated particles of the prior art do not develop their full strength until the resin coating has cured in the formation in the event of rapid closure of the fracture the proppant could be crushed before the resin cured resulting in decreased permeability. The present invention through the use of a dual resin coating eliminates this problem.